text:
Is it legal to shoot down hijacked passenger aircraft in some countries?
Are there countries which post-9/11 have adopted legislation allowing the shoot-down of passenger aircraft under any circumstances ?

Why do you think any legislation is needed? That is exactly what the United States was going to do to flight 93, even though the F-16 was unarmed, she was ordered not to let it hit Washington. The only reason it didn't happen is that the passengers did it before the Air Force could.

Sorry but both statements above are wrong. In countries governed by the rule of law you cannot rightfully violate the law. Nobody is above the law, not even the military, police, politicians or ministers. (Example: They murder someone on the street, they'll get prosecuted.) What the military can and cannot do is codified by law. The Constitution is part of a country's legal framework. You can't shoot down innocent passengers from the air if its against constitutional law to kill people (which is standard). Permission to kill citizens has to be granted by law. Otherwise it's not a nation of law

To add: In many countries the military is not even allowed to operate domestically. Exceptions (if any) are very narrowly defined and mostly include disaster management only – not killing civilians at any rate. Hijacking could be added as such an exception by law, but I am unaware that such a law has actually been passed anywhere. Hence my question.

Why are you even asking the question if you are saying that some semi-answers by others are wrong?

You might get a better response on law.SE. The aviation aspect of this question is minor compared to the legal and political ones, in my opinion.

re: "You might get a better response on law.SE" – thank you. But legal questions about aviation topics are on topic here and even have a tag, so I don't understand why this should not have a place here. Shooting down of passenger planes is a topic that could not be more squarely within the aviation world.

@SMS: I don't see why the question if some countries adopted such a law shouldn't be asked because of outlandish claims in comments (that killing innocent civilians doesn't have to be regulated by law). @ reddHerring: Your example makes my point actually, because it IS legally codified – as you say so yourself: "quite legally". Deadly force is allowed by law in some instances, but it HAS to be allowed by law. Otherwise it is illegal. Obviously.

@SMSvonderTann from his other related question, he just seems to want his own opinion that there is never a case for destroying an aircraft in flight during a hijacking confirmed.

Yes, I don't see this case, but this is immaterial for the question at hand if such a law has been adopted.

Partly related: Can a general aviation aircraft be forced to land when crossing the airspace of a country?

This isn't an internet forum.  If you want to argue about this, take it to a forum.

@summerrain.  Your comment about their being a legal tag is very valid, but the moderators don’t care about the tags. They prefer to not have questions, and define what is “in scope” by the pathetically narrow topics in the help centre. Using that logic, all the questions on gas turbines are off topic, which shows how flawed it is. I’ve tried debating this, and it gets no where. So, we don’t bother answering questions much anymore. But the moderators often slip up. I once even answered a question about a tank engine in aviation.SE!

There is german stage play and movie about this dilemma. Near the end there is a break, we’re the audience is asked to judge about the accused pilot of the eurofighter typhoon - the end is altered depending on majority vote. I like it because it’s interesting and the pilot itself is neither evil nor a sunny boy. Beware that german law doesn’t have a jury and doesn‘t judge about good/bad but about lawful/unlawful. It’s available on Netflix, probably without vote-feature and sadly only in german, but also no exaggerated hollywood clone: https://de.wikipedia.org/wiki/Terror_%E2%80%93_Ihr_Urteil

@Penguin: Put a parachute on the tank, throw it out of a plane...it‘s aviation?

@Peter. I don't think a tag exists in aviation.SE for that! {So, in that example, it's certainly out of scope!!} Cheers.

Yes it's legal in some countries/locations.
In the US after Sept-11 Operation Noble Eagle expanded the US Air Force's role to providing air defense for the entire US domestic airspace. Flying into a Special Flight Rules Area (or whatever they are calling it this week!) such as that over Washington DC and or into the even more restrictive Washington, D.C. Metropolitan Area Flight Restricted Zone (DC FRZ) without adhering to the Zone's rules and failing to respond to communications and intercept protocol can (in theory) result in being shot down. The DoD (understandably) doesn't reveal details on the exact rules of engagement or who has the authority to order such an action (believed to be POTUS, Sec. Def and Deputy Sec. Def) but it's definitely possible (and legal), and they've come close. While the near-downing in the link was for a private aircraft the same rules would allow for the shooting down of a hijacked commercial aircraft.
Internationally speaking.. well it's a grey area. As discussed in this paper there is no International Law that specifically prohibits doing so - although it's generally seen  as a bad thing to do except as a last resort. So as Ron Beyer mentioned in a comment, there generally doesn't need to be any legislation enacted to do so - because there isn't any that prevents it.

"there is no International Law that specifically prohibits doing so", what is an "international law"? Laws are country defined (except EU "regulations"). There are treaties though.

@mins Wikipedia does not agree with you.

@DanilaSmirnov: You have a point, but "International treaty law comprises obligations states expressly and voluntarily accept between themselves in treaties" and "International law is consent-based governance. This means that a state member may choose to not abide by international law, and even to break its treaty." They apply to governments and are bilateral/multilateral. Treaties are not comparable to ordinary laws, as there is no possible prosecution: Immunity from prosecution

@mins while I don't disagree that the way international "law" works isn't a direct equivalent to ordinary laws and that really it's just another name for a treaty it's still a commonly used term, there is still precedent for prosecutions and it felt relevant to the question IMHO


text:
What is the vertical separation minima between IFR and VFR?
ICAO Doc 4444, 5.3.2 says

The vertical separation minimum (VSM) shall be:
a) a nominal 300 m (1 000 ft) below FL 290 and a nominal 600 m (2 000 ft) at or above this level, except as provided for in b) below; and
b) within designated airspace, subject to a regional air navigation agreement: a nominal 300 m (1 000 ft) below FL 410 or a higher level where so prescribed for use under specified conditions, and a nominal 600 m (2 000 ft) at or above this level.

Is Vertical separation minima between IFR and VFR 1000 ft? If yes, then should a VFR flying at VFR cruising level be given climb/descent or vectored to have lateral separation?
No United States specific answers please

ATC will try to provide separation for known VFR, but if the airspace is shared with uncontrolled VFR aircraft, it would be impossible to separate VFR and IFR aircraft.

When a seperation service is applied those are the vertical seperation standards applied. The only different between IFR and VFR is the class of airspace and if a serperation service is applied.

Class
Type of Flight
Seperation Provided

A
IFR
All Aircraft

VFR not Permitted

B
IFR
All Aircraft

VFR
All Aircraft

C
IFR
IFR from IFR; IFR from VFR

VFR
VFR from IFR

D
IFR
IFR from IFR

VFR
Nil

E
IFR
IFR from IFR

VFR
Nil

F
IFR
IFR from IFR as far as practical

VFR
Nil

G
IFR
Nil

VFR
Nil

Extracted from ICAO Annex 11: Air Traffic Services, Chapter 2, Appendix 4
The vertical seperation standards are then applied where ever speration is provided to the aircraft.

I admit I am not an expert, but having skimmed through Doc 4444 I see nothing that provides other vertical separation minima applicable only to VFR aircraft. Therefore I must assume that the minima you listed apply equally to IFR and to VFR aircraft, and you are correct that lateral separation must be effected if vertical separation does not exist.
However, those minima only apply within that airspace in which VFR aircraft are provided separation services at all. From 5.2.1.1:

Vertical or horizontal separation shall be provided:
a) between all flights in Class A and B airspaces;
b) between IFR flights in Class C, D and E airspaces;
c) between IFR flights and VFR flights in Class C airspace;
d) between IFR flights and special VFR flights; and
e) between special VFR flights, when so prescribed by the appropriate ATS authority;

See also the Note at 5.10.1.1:

Note.— Pursuant to Section 5.2, but subject to certain exceptions stated therein, ATC is required to provide separation between IFR flights in airspace Classes A to E, and between IFR and VFR flights in Classes B and C. ATC is not required to provide separation between VFR flights, except within airspace Class B. Therefore, IFR or VFR flights may constitute essential traffic to IFR traffic, and IFR flights may constitute essential traffic to VFR traffic. However, a VFR flight would not constitute essential traffic to other VFR flights except within Class B airspace.

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text:
Is there a pressure rise and velocity change across a turbofan fan and stator?
I've been looking at the pressure / velocity / temperature diagrams across jet engines. These have really helped me to understand the different roles that the compressor/turbine stator and rotors play, and how they compress and then extract energy from the gas flow. I then started thinking about the fan. I've done some reading on how propellers generate thrust in the same way as wings do, with the high and low pressure areas. My question is, do fans primarily generate thrust similar to propellers (i.e. with the high and low pressure areas), or do they operate more like the rotor and stator set-up, with the interaction of subsequent blades acting like nozzles to increase velocity?

What do you mean with the fan stator?

@Koyovis - as you clarify below, the fan exit guide vanes. So I think my real question is, do the exit guide vanes transform internal energy of the air exiting the fan into kinetic energy like the turbine stator nozzles. However, the terminology of "exit guide vane" I assume parallels the intended use of the inlet guide vane, in that they change the direction of the air only - there is no change in energy form.

Sure: https://aviation.stackexchange.com/questions/19664/what-are-these-strange-blades-behind-the-n1-fan-in-a-boeing-747-8-engine

Also many discussions on airliners.net etc.

The answer is kind of yes and no.
Actually the thrust is always the force with balances the pressure forces on all the parts of an aero-engine.
This means (clarification in the next paragraph) that both propeller and jet-engine "create" thrust with the pressure and suction side pressure distribution of the blades. The following picture from Rolls-Royce's The Jet Engine illustrates that nicely.

Actually nozzle and turbine create drag, because the pressure forces in them do not point into the direction of flight. (Thought-Experiment: Just imagine the bolts of the nozzle would fail: If the nozzle would create thrust it would stay attached to the engine. But as one can imagine the nozzle would fly away from the aircraft).
The nozzle is needed to control the pressure level of the whole engine in order to allow for the engine-process-cycle to work properly. And because of this link one can use momentum conservation to calculate the thrust based on the inlet and exit momentum (velocity) of the airflow.

Looking at a turbo-fan engine the answer stays the same.
From the turbo machinery point of view: The fan does the same two things every compressor stage does. Two things: adding static pressure and whirl. The whirl is converted into pressure in the guide vanes, downstream of the fan.
From the thrust point of view: The fan-blades create the thrust, the stator-vanes (OGVs) down-stream of the fan-blades create drag. But as with the compressor and the nozzle above, without the OGVs the pressure-ratio of the fan would be lower and so would the fan-thrust.

One additional part is needed: The nozzle of the bypass. The above picture by JAXA nicely illustrates that (at least for modern engine installations) the bypass airflow is guided through a convergent-divergent nozzle.
So to sum up. Yes there is a pressure rise (at least locally) across the rotor and a velocity change as well. But the thrust (and drag) is produced by the pressure forces on blades and vanes (and all other engine parts).
And yes the fan operates very similar to a rotor stator setup of a compressor.
But the vanes in a compressor work like a diffuser, reducing the flow velocity. In order to keep the flow velocity mostly constant the annulus is shaped like a nozzle to balance the increased density.

And now for a paradox: Let the bolts fail and the nozzle fly off: Without it, the engine will create less thrust.

@PeterKämpf: true, you might have added a "Spoiler-Alert" before the paradox. But of course you are right, without the pressure reduction in the nozzle the turbine would expand until it reaches ambient pressure which then would reduce the pressure (enthalpy) difference in the turbine requiring a lower pressure rise in the compressor which results in lower thrust.

@mins: True, I'll edit that, I focused on the question in the text and forgot about the question in the title, thanks mins.

@rul30 thank you for the very informative answer. It really has added a lot to my understanding of the thrust/forces through the core. What I was intending to focus on - with my poorly worded question - was the N1 fan at the front of the engine. I'm really curious in the airflow dynamics as it enters the duct, passes through the fan blades, then through the exit guide vanes.

not sure what caused the numerous down-votes. I am happy to clarify or change my answer, but without any comments that's hard.

Added clarification what I mean by outlet guide vanes, thanks @Koyovis

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text:
How to navigate an aircraft without GPS?
In case of loss of GPS: how can an aircraft not only be navigated but navigated so precisely that it can pinpoint its destination airport ?
(case in point: Finland and Norway are telling airline pilots to be ready to fly without GPS )

Before 1974, aircraft navigated without GPS and were able to cross oceans.

@757toga The "1974" figure was just an example of when the Air Force first launched Block 1. GPS wasn't even certified until 1984 for use in IFR. There are quite a few GA aircraft that don't have it, and 10's of thousands of smaller ultra-light or even LSA's that don't. I'm not sure about commercial aircraft though, I would think by now all of them would be equipped with some kind of GNSS.

@757toga Same here, just adding info, no worries. I'd still be surprised though if they didn't have GNSS, mostly because the ADS-B mandate will affect all larger aircraft types as they typically don't hop around between Class-D/E and use A/B/C.

Simple, you look out the window :-)  Then you have a compass, VORs, and all that.  Personally, I've been flying since the late '70s, and never used a GPS.

The question you should be asking is rather, how did airliners navigate before the widespread adoption of satellite-based navigation systems such as civilian GPS?
One can navigate by way of landmarks (as is typical in low-altitude VFR general aviation), but more typical in commercial aviation in the absence of GPS would probably be to navigate by either inertial navigation, or by radio beacons. There are or have recently been several systems that can be lumped together as "radio beacons", such as for example NDB, VOR/DME and ILS, which are used for different purposes, and have different strengths and weaknesses. Until recently, another such system was LORAN.
Major airports typically offer a number of radio beacons that can be used for locating the airport, and there are normally waypoints, sometimes associated with radio beacons of their own, along which a route can be flown even without external references (such as in instrument meteorological conditions, IMC). 757toga's answer to this question shows what such a navigation system for long-distance flight can look like.
GPS does make it easier to fly the most efficient route possible under current conditions, but it is hardly required in order to fly some route from one airport to another. Being prepared to fly without GPS might, in the simplest of cases, simply mean that you plan for a slightly less direct route and treat your non-GPS navigational aid equipment as required, possibly even primary, rather than secondary or nice to have, since you know there's a clear risk of losing GPS through no fault of your own or of the onboard equipment.

Does LORAN-C fit in there as well?

In addition to the rather short range radio beacons you mentioned there was the LORAN long range hyperbolic radio navigation which might also be worth mentinoning.

Good point; edited. The list was never meant to be exhaustive, but I agree that LORAN makes a good addition.

It’s rare but there are a few small aircraft left with LORAN receivers.  It was the rage in the late 1970s and early 1980s but never achieved a large place in general or commercial aviation.  By the mid 1990s GPS was the default new navigation system for military use and quickly acquired the civilian market when access was allowed.

@757toga Might be a shortage of LORAN transmitters, too, since the USA and Canada turned theirs off in 2010 and much of Europe deactivated theirs in 2015.

Well, I fixed up the answer a bit further. Hopefully, the current revision of my answer doesn't suffer from such problems.

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text:
Does wind provide lift for hang gliders in the air?
Basically, if there are no thermals, but a strong wind, is there a way to use that wind to gain altitude?

Quite possibly, if there is a hill.

Or if you can throw out an anchor.

You need some gradient, like in the boundary layer of the atmosphere. Then yes, but it is very stressful.

Yes, it's called ridge lift. From Wikipedia:

Ridge lift (or 'slope lift') is created when a wind strikes an obstacle, usually a mountain ridge or cliff, that is large and steep enough to deflect the wind upward.
If the wind is strong enough, the ridge lift provides enough upward
force for gliders, hang gliders, paragliders and birds to stay
airborne for long periods or travel great distances by 'slope
soaring'.

https://en.wikipedia.org/wiki/Ridge_lift
If you happen to be in the San Diego area on a nice summer day. Go to the Torrey Pines glider port above Black's Beach in La Jolla and just north of the Scripps Institution of Oceanography. The onshore wind striking the bluff overlooking the beach is forced upwards and provides plenty of lift for hang gliders working their way back and forth.
You can see images of people doing that at the Torrey Pines Gliderport website or just Google "torrey pines gliding images" for additional images.

Thanks.  However, I was more wondering whether it can do so over a flat area, without the deflection of a ridge or obstruction.

@Theo -- is there a particular flat area that you have in mind?  I ask because some early glider pilots would fly nearby to oceans to take advantage of the natural convection currents that provided some lift.

But it's not the wind per se, it the fact hat the wind, hitting the ridge, creates an UPDRAFT. It's the Updraft that provides the lift.

There is another way, that of harvesting energy from the wind gradient. Some birds (and some sailplane pilots) use it. It's called 'dynamic soaring'...

Better call it "dynamic soaring"; without "dynamic" the word is used for flying in thermals. But you better have the physiology of an albatross to sustain the dynamic loads for an extended period of time.

True. The correct name is 'dynamic soaring'...

FWIW there is a huge amount of energy that you can extract with dynamic soaring. In the world radio control, dynamic soaring gliders are the fastest airplanes you can fly - faster even than jet turbine or rocket powered RC airplanes.

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text:
Why do Airbus aircraft typically have less wing flex than Boeing aircraft?
I was watching this video on YouTube about Boeing vs Airbus aircraft, and at around the 5:09 mark the female pilot stated that Airbus wings are much "stiffer" and "less aerodynamic" compared to Boeing's' more "aerodynamic" wings.
I looked at a few videos and it does appear that Airbus aircraft have some wingflex but not to the same extent as Boeing aircraft.

Taken from: this video
I gather that wing flex is mostly beneficial for aerodynamics so why then, do Airbus aircraft typically have less wing flex than Boeing aircraft?

"I gather that wing flex is mostly beneficial for aerodynamics" - no, it isn't. What makes you think that?

It's not a super scientific statement. There may be some half-truths referring to half-truths inferring that some design is better, just for show business sake.
"The Boeing wing is way more aerodynamic than the Airbus wing" is not something that has any value at all. All wings are by definition aerodynamic, there is no sliding scale. All wings are the result of a design process, balancing many input parameters. If an aircraft manufacturer cannot get the aerodynamics of a wing right, they have no place in the aerospace industry. Both Airbus and Boeing make very fuel efficient and safe aircraft that compete head to head in the market place.
Wing flex is one of these things that the structural engineers must cater for, after the aerodynamicists have had their go at the external shape: the planform, aerofoils etc. It can be considered a secondary design parameter that:

provides dihedral in flight, for stability when banking;
acts as a spring against turbulence;
can cause grief with flutter;
may provide difficulty for the ailerons to operate.

For a given wing shape, wing flex is the result of the ratio of strength and elasticity of the wing material. Composites are the latest in aircraft material, being lighter and stronger than aluminium. It is five times stronger than aluminium, and flexes about half when under load. But because it is stronger a lot less material can be used to make the wing lighter - even though a certain amount of carbon fibre flexes less than the same amount of aluminium, there is simply less of it. The ratio of strength over flex of carbon fibre is much higher than that of aluminium.
All aircraft with wings made of carbon fibre flex a lot more than aircraft with wings made out of aluminium. Both the A320 and the B737 have aluminium wings. For a given material and wing area, wing flex is a function of

Aspect ratio: a long, slender wing flexes more than a short, stubby wing. Or the long aluminium wing of a large airliner flexes more than the shorter wing with the same aspect ratio of a smaller airliner.
Wing thickness. The thicker the wing profile, the less sweep angle needs to be used to delay drag divergence. This is actually a sign of an aerodynamically more advanced wing, and the thicker profile makes the wing bend less

Those sort of comparisons make much more sense than "the wing of a Boeing is much more aerodynamic than that of an Airbus". That's only showbiz.

I agree, although subjectively Airbus wings (from A320 / 330 era) might visually look more stiff than Boeings of the similar era (777 (I know it's a bit newer)), that alone can't be used to determine "aerodynamic" quality. Airbus uses their 30 years old wings even on today's built A320 / A320 Neo which tells me that the initial design was spot on.

777 and A330 wings have the same span, about 60m, so they would be directly comparable. Have not looked at videos of the flexing, but the flexing itself is not a measure of quality of the wing.

Re-designing and re-certifying a wing will cost a lot of money. If you touch at the geometry and performance of the plane, the certification plan is not the same than if you keep it as is (case of CEO to NEO)

Wing shape ie planform, stiffness, thickness, chord length etc are driven by the design parameters that the designers are tasked with. The wingtip shape and the subtle changes in each of those parameters along the chord and span are dictated by the need to extract maximum energy from the air flow so as to minimise drag and maximise the lift. This is where science and a degree of art creeps in to maximise the yield. These parameters are to be true for the entire range of speed and mach number including low speed etc.
The Boeing 737 series has a more conventional high speed wing whilst the A320 has a supercritical wing with their respective benefits. Flexing of wings are an aero-servo-elastic phenomenon that can be tailored in composite wings and to a lesser extent in conventional material wings. It's a finely orchestrated task between the aerodynamics, structural, stability & control, materials and weight management engineers to optimise all of the above.
Needless to mention the increments in performance are limited to single digits only and so is the differential between Boeing737 and A320 wing performance.

+1, good answer. Yes wing re-design is a huge cost driver and all aircraft manufacturers will try to find solutions that won't necessitate this. To use wing flex as the one and only criteria for "aerodynamic design" as the pilot mentioned in the OP is not useful at all.

Yes, yet 737 NG had its wing redesigned, and even the Classic had a supercritical airfoil.

@ Zeus Iit was overdue for the 737, wing design had advanced a lot since the 60s.

I get what you are referring to. I have carefully observed it myself.
Airbus A320 is a popular aircraft, when the flaps are fully extended, the wing and flap together has a sharp angle.
On the other hand when a Boeing 737 has its flaps fully extended, the wing overall has a very smooth curved shape.
To a layman, the Boeing wing definitely seems more aerodynamic. But that's only what we infer from the shape.
I have friends who work for both Boeing and Airbus. All of them talk about how they spend a LOT of time optimizing and re-optimizing their designs. Unfortunately they design the cabin not the wings.
I'm pretty sure in the wind tunnel, both wing designs offer similar lift and drag. Hence their  lack of interest in fixing something that works well.
Airbus is famous for retaining things that work. The infamous barking noise of the hydraulic pumps is well known. It has worked for so long that it's "side effects" can be ignored.
Here the "side effect" is the wing looking sharp, weird and stiff. As long as the drag and lift values are fine the business does not justify the cost of modification.
Another important factor as stated above, is the structural stability. The wing must undergo least vibration, less stress and strain for it to last long.
The shape and structure of an Airbus wing may require it to be shaped such that stress is minimum. Or maybe the Boeing wing was designed like that. We can't know since both manufacturers are not that open to sharing such details to the public.
The stiffer looking Airbus wing requires less moving parts and may offer better structural integrity. That too cannot be stated as a fact by me.

I get what you are referring to Are you sure? Wing flex is defined as the bending of the wing due to aerodynamic loads, not the extension of flaps or other aerodynamic surfaces.

Welcome to aviation.SE! If you're new to the site, you might find the [tour] is useful.


text:
What kind of flaps are used in gliders?
I'm estimating some wing parameters for an imaginary airplane (simple college project). We are allowed to use coefficients for any basic types of flaps (plain/split/slotted/Fowler etc), but since I'm doing a glider I'd like to go with the one that's really used.
tl;dr - What kind of flaps are used in modern gliders (preferably training ones)?

modern training gliders (like an ASK 21) do not have flaps. Flaps are used on high performance gliders because the pilot needs some more experience to operate them safely.
Gliders, especially training gliders control sink speed by spoilers.

Image source: https://de.wikipedia.org/wiki/Luftbremse#/media/File:Alexander_Schleicher_ASK_21_2.jpg

Wow, this is really amazing, thanks.
Since I might have to use some kind of flaps to calculations, could you please explain briefly which flaps are used for these high performance gliders?

@rafal.sz usually plain flaps are used because they are simple to construct need low maintenance and are easy and quick to operate manually through stakes.

Flaperons are quite common as far as I know. They are, in contrast to powered airplanes, often used in upward pointing position to improve glide ratio in higher speeds.

@Matrin Do you have an example where it is used on a glider?

Yep. I'll write more detailed answer later this day when I have more time to collect some resources.

As Timothy already mentioned, most trainers do not have flaps, but when a glider is equipped with flaps, it would be most commonly in form of flaperons where full-span ailerons are coupled with flap control allowing common-mode motion both upwards and downwards. In contrast to the powered airplanes, flaps in gliders are used not only for landing, but also (maybe more importantly) to adjust optimal glide ratio for the speed currently flown. Thus upward turned flaps are common configuration during fast gliding between thermals for example.
Finding information for particular gliders, especially such mentioning flaperons explicitly turned to be quite hard. Pilot operating manuals usually just describe how to manipulate flaps handle without going into the details on actual construction. Typical placecard for flaps handle (from DG-400 manual) does look like this. Ranging from landing configuration to negative flaps.

Particular gliders equiped with flaperons include (maybe not the most modern ones, but those I was able to find some more info about, good enough as examples IMO):

DG-800 and LS-10 has full-span flapperons. Mentioned explicitly in the flight manual: The wings feature single piece flaperons, which are driven at two places. The mixing of aileron and flap deflections takes place in the fuselage. (The same formulation exists in LS-10 flight manual.)
DG-600 -- according to the wiki.
ASW-20 (and probably some other Schleicher gliders too) -- there are two distinct surfaces on each wing, sometimes referred as "flap" and "aileron", but both of them actually moves according to the mixed inputs of both controls, the ASW-20 flight manual contains nice table of deflections:

Eta has three flaperon sections on each wing, each with different mix ratio between control inputs
... and many others, I believe


text:
Why and how can a fume event occur?

« A fume event occurs when bleed air used for cabin pressurization and
  air conditioning in a pressurized aircraft is contaminated by fluids
  such as engine oil, hydraulic fluid, anti-icing fluid, and other
  potentially hazardous chemicals. » (source)

Wikipedia does not explain how and why a fume event can occur, but notes:

« The cabin is pressurized with bleed air tapped from the jet engine's
  compressor sections, which are prior to the combustion sections. » (source)

Question:
(1) status quo
Given that bleed air is tapped at an early stage before coming into contact with fuel and thus cannot come into contact with fuel, why then can it come into contact with the other fluids quoted above ?
(2) solution?
To avoid any potential contact with these fluids, couldn't it be tapped an an even earlier stage (in the front section of the engine) or rerouted/transported differently from the engine into the cabin? Isn't there any solution (other than bleedless aircraft) to eliminate the potential for both fume events (acute exposure) and long-term health effects (chronic exposure) for the cabin-crew and frequent flyers due to long-term exposure to minimal doses ?

(1) How can can other fluids contaminate the bleed air?

Engine Oil:
The engine fan and compressor stages are rotating pieces of metal on metal and thus need lubrication. Engine oil is used for this purpose and could come into contact with the bleed air in case of a leakage (see mooveaviation.com for details).
Hydraulic Fluid: Some components of the engine, e.g. the thrust reversers, are hydraulically actuated. Again, a leakage in a pipe could lead to hydraulic fluid entering the engine.
De-Icing Fluid: When an aircraft is deiced on the ground, the crew needs to disable the bleeds during the deicing in order to avoid any contamination, see e.g. Boeing 737 NG FCOMv1 XP.16.8 (Supplementary Procedures - Adverse Weather):

CAUTION: Operate the APU during de-icing only if necessary. If
  the APU is running, ingestion of de-icing fluid causes
  objectionable fumes and odors to enter the airplane.
  Ingestion of snow, slush, ice, or de-icing/anti-icing
  fluid can also cause damage to the APU.
[...]
Engine BLEED air switches ......................................OFF
  F/O
Reduces the possibility of fumes entering the air conditioning
  system.
APU BLEED air switch .............................................OFF
  F/O
Reduces the possibility of fumes entering the air conditioning
  system.

Some of the deicing fluid might flow into the engine afterwards and if the bleeds are turned on to early, the fumes might get into the cabin.

(2) Could the bleed air be tapped at an earlier stage?
Bleed air needs to have a high enough pressure to be usable for pressurizing the aircraft at cruise altitudes. Moving to an earlier compressor stage would reduce the available pressure. Some engines have bleeds in both the low and high pressure compressor and will automatically use the appropriate one depending on engine thrust and pressure.

In many aircraft hydraulic fluid can also leak into the bleed air ducts later. The bleed air is used to provide head pressure to the hydraulic system, so the pipes are connected. There are check valves, but if the reservoirs are overfilled or the valves are deteriorated, some fluid can leak through them to the bleed air system.


text:
If all aircraft were TCAS-equipped, how would this change ATC routines?

« The primary purpose of ATC worldwide is to prevent
  collisions, organize and expedite the flow of air traffic, and
  provide information and other support for pilots. » (source)

If all aircraft were equipped with TCAS, in what ways (if any) would this change ATC operation ?

@StephenS: This would be a good standalone answer, if you want to add it. Judging from your explanation, ATC operation wouldn't change at all, correct?

It would not change the ATC system at all, since TCAS and ATC work on completely different levels and have vastly different purposes.
We even have an ICAO recommendation clearly stating that ATC procedures should not be changed based on ACAS implementation:

The planning of air traffic services and the determination of the level needed for these services should not be influenced by considerations relevant to the deployment of ACAS in a specific airspace.

ATS Planning Manual, Doc 9426
(Note: ACAS is an umbrella term, covering different types of airborne collision avoidance systems, the most common of which is TCAS)

Think of the ATC system as the road network, traffic lights, road signs, and police.  TCAS is a standalone device in your car that warns you the guy coming at right angles to the intersection in front of you is going to run the light and T-bone you.  Whether you have such a device or not, it doesn't change how the traffic control system work.
TCAS has been around for 40 years and it didn't change the way traffic is controlled because it's a stand alone system that only impacts the crew on board the airplane that gets a TCAS warning.
TCAS is mostly to protect big aircraft from incursions from small aircraft, that are hard to see and that ATC can't always prevent. What got it going in the first place was the famous crash in LA where the DC9 on an approach overran a C-172 back in the late 70s.

+1 That's a good explanation. So is your answer, in conclusion, "It wouldn't change ATC operation one bit" ? If yes, please add to your answer. If no, please explain what would change (from the point of view of ATC). tx


text:
Special Purpose Tablet for Commercial Navigation app
I just posted a request for advice on how to find right android tablet to integrate commercial flight navigation app (e.g., iFLY) to avoid bloat, OS updates that may not be compatible, and add future WiFi integration with commercial ADS B-In data overlays (e.g., FlightBox, etc.) on the Geo-referenced moving map in the navigation app.
(Please help, I don't know how to send this request to Don Hulme.)

Welcome to aviation.SE! Unfortunately, we don't usually answer 'shopping' questions: they're often very subjective, and answers can change quickly over time. You might like to take the [tour] if you aren't familiar with how this site works, and/or bring this up in [chat].

I assume you mean me but I don't know why you would want my advice. I don't use a tablet for flying so I don't have anything relevant to recommend. You can't direct questions to specific people because the idea is that anyone who knows the answer can post it.

I wish this site would embrace this topic because it is simply an extension of aircraft avionics and navigation.
This question will probably be closed, but I think this is an important question that doesn't have any definitive answer on the internet. I recently went through the upgrade from paper charts to electronic flight bag. There is very little guidance (information overload) or standardization on this topic. However, it is helpful to look at what some of the large aviation operators use.
Several flight schools have specific recommendations for tablets. For example,
ATP Flight School recommends the iPad mini, 32 GB, and Wi-Fi + Cellular for it's flight instructors.
Directly from ATP:

ATP recommends the iPad mini, 32 GB, and Wi-Fi + Cellular (Available
  from Apple).

I've seen similar recommendations from other flight schools and Universities with flight programs but at the moment can't find the references (will update once I locate).
Personally, I use an iPad pro because I was informed that it has better anti-glare properties on its glass than the non-pro version of the ipad, which is important for day operations in the cockpit.
(Note, Apple lists antireflective coating on both the Pro and the Mini version of iPad but not the regular iPad).
I have run several of the Navigation apps on android and iPads and noticed that they seem to be a little better on the iPads. Some even have more features  on the iPad versus the android (i.e. WingX Pro).
Also, 32GB is more than enough (as of 2018) to run the navigation apps with all charts loaded and everything turned on including synthetic vision, assuming that it is a dedicated flight tablet and you don't have it loaded down with a bunch of other content such as videos, songs, pictures, etc. Also, you'll want to install just one navigation app, if you install multiple, and download all the charts for each one, it duplicates all the data and will use up your 32GB quickly.
I have ADS-B external antenna and it works just fine with either. The ADS-B I used creates a local wifi signal that the tablet connects to the antenna and all of the traffic and weather data is incorporated into the app. In other words, ADB-B data should work with any tablet that has wifi capabilities.

As you said, there's no "definitive answer" and that's exactly why we've avoided questions like this so far. The OP is asking for opinions ("avoid bloat") and speculation ("OS updates that may not be compatible"), among other very subjective points. But having said all that, I do see your point that this topic is valuable for many people, it's just that's it's too difficult to give a single useful answer. If you think there is a good way to handle these questions then it would be great if you can bring it up in meta.

Perhaps OP can modify question. I tried to craft my question to point to objective points such as what do major flight schools recommend, technical requirements (memory), and physical qualities of the instrument (anti-glare). Or perhaps someone can help OP by adjusting question to be more objective.

In addition, OP asks for recommendations on Android tablets, and your answer seems to recommend "get an iPad." Based on what you wrote that may be the better choice but it still seems like a bit of a non-answer.

@fooot agreed. However, I was in OPs position about a year ago. I wanted to make it work with an android tablet, but after much research and trial and error I discovered (to my chagrin) the industry standard appears to currently be the iPad. Another point which I should add to my answer is to get the cellular iPad even if you don't activate it because the GPS that comes with the Cellular iPad has GPS+GLONASS, which is excellent for a moving target such as an aircraft.

Devil07: Seeking a "pure android" OS with built in GPS and WiFi. iFLY app from G-Play meets my requirements whereas WingX for android device does not. The popular android tablet I used to evaluate the two apps did not permit deletion of android 7.0 after it was downloaded. ADS B In boxes are available, but first need to fly with my own tablet + Nav app.

@AQuillen is there a reason why it must be "pure android?" You may want to try contacting the developer of the app you are using, and ask them for list of compatible android hardware. Then use that information to narrow your search. Or, if you know the technical specs that you require of the "pure android" tablet you could go to a IT related forum and ask for guidance on tablets with your specific technical requirements. Good luck!


text:
Does an overhaul reset the 'life limit' of a turbine engine?
I've come across a very unclear maintenance document.  It's often referred to as a "Hard Card" for an Engine Axial Compressor Turbine.  This specific part happens to come from a French manufacturer, so there may be a language barrier issue involved, but I'm trying to determine if anyone has clarified documentation that discusses this issue.
(Source: helisupportnz.com)
On this card, beneath a section translated as "Available Life", "Module Life Limited Parts", and "Usage Limited Parts", there is a section translated as "Time for the Calendar Limit".  Next to this is another section translated as "Date for First Entry into Service" and a logical third calculated section translated as "Limited Date for Usage".
These connect with each other very clearly and easily.  For example, one card marked with a 15 Year Calendar Limit is showing a Limited Date for Usage of 2020 when the Entry into Service was 2005.
I have a maintenance technician who is under the impression that this Calendar Limit is reset after every Overhaul of this Engine Module.  Unfortunately, no where in this card does it say that this is the case, and if this module gets reset when it is actually a Life Limit, then we will run into an overfly.
Has anyone come across this and received a clear answer from someone in authority over this?

Is this something that can be clarified with the manufacturer? It isn't a bad question, but it is extremely focused (axial turbine for a French unspecified manufacturer) and we may not have the diversity/experience to provide an answer on this.

I second Ron. That really needs to be verified with the manufacturer. I wouldn't bet a safety violation on what somebody here has to say.

Essentially the only way a life-limited part's life limit can be reset is by melting it down and using the metal to manufacture new parts.

All nations operate with very similar rules. For the US, the short answer is nearly all turbine powered aircraft operate under part 119/121 (air carriers) rules. Each company has maintenance and operations specifically tailored and tied to their operating certificate and so it depends on the approved procedures of that companies operating certificate.
In your example, no mention is made of the part being "rebuilt", therefor the use of the part will continue where the previous use left off. The card says it is a "life limited" part so it's use will run out according to operating hours, calendar time from first installation, or number of operating cycles, etc. The parts life span is set by the mfg or operating certificate rules.
For example if a part  has a 7yr calendar "life limit" (like some seat belts) and the part was installed January 1, 2015 then removed 10 days later, the part would become worthless Jan 1, 2022 even though it was used less than 1 month - the clock starts ticking from it's first use.
This card says the part has a 15yr calendar limit which will usually be in addition to other limits. It will either be limited from it's first date of use, or it's date of manufacture (whether the part is used or not). Once again, exactly how it should be interpreted will be based on the mfg or operating certificate rules.
Most life limited part life spans are TIS (time in service) and therefore expire after a certain number of hours of operation. If TIS for the turbine blade is 5000hrs and airplane "A" used it for 4000hrs, it could remain on the shelf indefinitely - be re-installed on airplane "B" and used for an additional 1000hrs. The card is designed to keep track of the dates and how long the part has been used - TIS, calendar, cycles, or whatever.
[I'm guessing for this particular part, but for many critical parts on helicopters and turbines a combination of TIS and calendar are used. So it would be something like 5000hrs TIS or 15yrs calendar from date of mfg - whichever comes first. Even if it sat on a shelf and was never used, it would have to be destroyed when the 15yr limit was up.]
NOTE: Due to a problem of expensive time limited parts being counterfeited, re-sold, or re-used, the FAA requires companies using life limited parts (once the service life is up) to make the part unusable by returning to a mfg, defacing, or destroying it.
Parts
On any US registered aircraft, "overhaul" and "rebuild" have very specific legal and maintenance meaning. "overhaul" per FAR 43.2(a) means "Disassemble, cleaned, inspected, repaired, reassembled, and tested to service limits", whereas Rebuild FAR 43(2)(b), 43(3)(j) means "disassemble, clean, inspect, repair, reassemble, and test to new part tolerances".
"Overhaul" means repair or fix the old part and does not change the wear or life of a part.
"Rebuild" means to make a "new" part out of the old one and the part's life is started over - "zero time". The part is treated as if it is a brand new part and many mfg even stamp a new serial number on the part.
Entire Engines
The above FAR's relates to "parts". For an entire engine, the FAA's AC-43-11 (Reciprocating Engine Overhaul Terminology and Standards) goes into great detail on what the terms mean and who may perform the work.
Why would AC-43-11 (entire engine) be limited to reciprocating engines? Because nearly all turbine engines are operated under Part 119/121 (not 91 or 135) operators. Part 119/121 receive FAA approval on a company by company basis and each company has maintenance and operations specifically tailored and tied to their operating certificate.
When a part 119/121 company has their operations approved, the FAA's standard practice is to hold them to all existing FAR's unless they show a "as good" or "better" way of maintaining the same level of safety. So, the spirit of AC-43-11 would be applicable unless some exception was given.

In my experience, engine parts with a life limit are thrown away when they reach that limit, and are definitely not reset by an overhaul (or repair). But... I have never ever seen a life limit in "calendar years". Normally the life is in engine hours, or throttle cycles. (See here for a description of cycles, and turbine blade failure modes.)
So, I would guess this life is for a static part, as I am unaware of a failure mode for a rotating part that is so loosely based on just being in the engine for x years.
However, without knowing what the part is, and what the failure mode is, I would be making a total guess if performing an overhaul, or repair, reset this life for this (unknown) part.

There are limits for "Calendar Life" based upon the degredation of a material's physical properties due to exposure to air, oil, ultraviolet life, fuel etc. Typically batteries in emergency devices, aircraft fabric, the fabrics used in life vests and rafts as well as the rubber used in fluid carrying hoses, orings used in propellers etc all have calendar lives
Flight hour life limits are typically there to set "inspection" times based upon flying hours.  Some parts may be subject to material property strength limits based on the number of flying hours accumulated and so an inspection is required to ensure no degredation has occurred. Typically the "flight hour inspection period limits are reset, however some parts may have a cumulative requirement for additional inspections to be accomplished as the part's flight hour life grows.
Cycle life limits are directly related to Material Fatigue and repetative stress cycles that the part's material is subjected to. These Material fatigue limits may be related to flexing where the material tensile strength is effected ans the material is work hardened thru repeated flex cycles and becomes brittle - resulting in cracks developing etc.
Cycle life limits may also be related to "thermal Stress cycles" such as the thermal stresses experienced by turbine engine rotating parts during engine start, shift from a low to a high idle, or from idle to take off power.
Thermal stresses also effect material strength and if the part's thermal stress cycles are not properly tracked and accounted for the part may suffer a catastrophic failure.
So, if a part that has any, all or a combination of these limits has it's "Life reset at overhaul" the end user may face an "unexpected event" that can cost them a lot of $$$$$$ to fix or defend against.


text:
Why does a plane fly like this?
Why does a plane fly like this?

https://www.flightradar24.com/LED5/dea0776

closely related: https://aviation.stackexchange.com/q/30443/1467

Thanks @Federico. It could look like some surveying - but I'm kind of confused since a lot of the traffic is above a small local airfield. Is the plane circling while waiting for the airstrip?

That's definitely not an holding pattern.

I checked flightradar because it sounds like a helicopter has been flying around the vicinity for a while. I am not sure if LED5 is a helicopter though. Is there any way to find out exactly what kind of aircraft tih is? It doesn't seem to go very fast.

Could be related: Blom Geomatics. No more than a guess. Corporate site. Nice view from cockpit of LN-NAB.


text:
How to find out what engine a plane had in the past?
I want to find out what engine a specific airplane had in the past.
For example for the Airbus A320. The sources which I've found tell me there were different engines. CFM56-5A, CFM56-5B, IAE V2500 etc. Let's say I flew in 1999 with Lufthansa. What engine did I most likely fly with?

Unless you have the tail number or you can determine that Lufthansa only flew a certain type in 1999 (on a certain route maybe) it will be hard to say. As you noted, the A320 has multiple engine types, and each type has different versions.

@Peter.  AirFleets.net is another database that has very similar info to the answer below: http://www.airfleets.net/ficheapp/plane-a320-69.htm


text:
Is it possible to create a specific crash site at every airport?
I have found the need to bring to your attention the possibility of creating various crush sites and the various airports for any unpredictable events that could possibly lead to a crush.  I believe that in this 21st century of our developed world we can possibly put together the technology than can hold the impact of an aeroplane which had lost total control of its engines and the last resort is to crush... Now the question is how safe can that pilot crush that flight or is there a way for the pilot to safely crush the flight with a greater possibility of survival.

It already exists, it's called a "runway".

Possible duplicate of Even after years of research, why are planes unable to keep passengers alive in case of a fiery crash?

If the plane is in such good shape that the crew can direct it precisely to a specially designated spot, why is it crashing?

In short, No
But this does warrant a bit of discussion in the spirit of a complete answer. First off the chances of losing all your engines on a commercial airliner these days is pretty slim. That being said it has happened and when it did the pilot was able to make a successful glide to a landing and whats important here is that the aircraft remained controllable. Similarly when no runway is within gliding distance forced ditching has been shown to be successful recently. In both of these cases there was no need for some kind of forced crash site. In both cases there were 0 fatalities.
In the event of a fast landing or a situation where perhaps the gear does not deploy properly or the aircraft no break system, many major runways are now equipped with EMAS to arrest the moving aircraft. This may be as close as we have to what you are asking about.
Unfortunately the major limiting factor in many accidents is loss of aircraft control either due to pilot error or aircraft malfunction. In these cases an impact zone provides no value as there is no way to get the aircraft there in the first place.

no break system I'd like to think there are no systems aboard an aircraft to purposefully break it. Maybe for when the chemtrail tanks are exposed? :)


text:
Is it always the case that any one STAR leads to all the runways?
I was making an addon for a flight simulator and I am having a problem figuring this scenario out:
Let's take CYOW - Macdonald Cartier international.
It has 6 runways: 04, 07, 14, 22, 25, 32.
It has 5 STARs : CAPTL4, DEANS8, LEAMY3, MEECH1, RIVER2.
For this specific airport every STAR can be used to approach runway 07,14,22,25 and 32. (Runway 04 isn't used for arrivals at all.)
I was wondering if this was the case for every airport or is it different? I'm not sure if my wording was right. I'll try to clarify it to the best of my abilities if further clarification is needed.

I think if you look at the example of FRA, some STARs are designed to lead to specific runways. In reality, however, ATC mix and match freely (making it a pain for flight crew to get the flight management computer reflect the actual clearance, as an aside).

No, that is not always the case. Many airports have different STARs from the same point going to different runways.

It all depends on the STAR, but generally the answer is yes.  You will have to refer to the specific STAR in question.


text:
Why does load shedding still occur in jetliners (e.g., 787) when there is so much power available?
I had a few questions about how electrical generator works on an aircraft, the 787 to be specific although I know it is similar for all aircraft.
So the 787 engines have 2 VFSG each which are rated at 250 KW each which is 1000 KW -> 1 MW which is said to power a 2000 population town. Since the power is generated using the rotation of the engine, im assuming it does not always produce 1 MW , only at peak rotations?
What on earth in the plane sucks that much energy, I know the 787 is a more electric aircraft, but still how much power does it really take? If the total power output of the plane is 1.45 MW, I'm assuming the APU Gens are able to provide 0.45 MW in total to the airplane.
If there is so much power available, why does load shedding still occur in aircraft?

I don't have a good breakdown of where all the 787 power goes, but the first couple slides here give you an idea how much power is required to replace what was done by the bleed air system on other aircraft and some of the typical loads.

@ZachLipton, actually, the slide that provides most concrete answer is in the middle.

Load shedding always occurs during engine start, according to the FCOM of the B789. Both engines are allowed to start simultaneously. The B789 engines are started using both VFSGs of each engine, which are mechanically connected to the N2 shaft via the accessory gearbox. So, the VFSGs are using power instead of delivering during engine start. The APU cannot provide enough power to prevent load shedding.
If the engines are started using external power, at least two 90 kVA external power sources are required. Optimal start performance is achieved using 3 external power sources (2 on the left forward fuselage and 1 behind the left wing on the fuselage). If only 2 external power sources are used, significant load shedding can occur (e.g. First Officers displays blanks and even the audio control panel receive and trasmit selections may be lost).
The APU is normally switched off during flight, unless e.g. one VFSG is U/S.
Load shedding should normally not occur when both engines are running and all VFSGs are operational (I haven’t experienced load shedding during flight).

First of all, I am not familiar with the electrical system on the 787.
In a modern twinjet airliners, if one engine-driven generator fails, load shedding is activate to protect the electrical system. What will normally be shedded is IFE and galley equipment (ovens, coffee makers, etc.). Should this happen, a checklist will direct flight crew to start the APU in order to establish normal power supply for all the equipment of the aircraft. If APU starts, the flight can continue normally. If APU is unable to start for some reason (flight could be dispatched with APU INOP, or APU generator INOP), then the crew will likely have to land at the nearest suitable airport, as the electrical reduncancy is lost.
So, in case 1 when generator fails and crew successfully starts APU, the IFE and galley equipment will not be powered for perhaps 5 minutes or so. No big deal to be honest. In case 2, where the APU is not available, the flight will have to divert to an airport, so no real need for ovens or IFE anyway.
Now, of course the manufacturers could design electrical systems that would handle this kind of failure, but the cost/benefit just doesn't support it. The generators would have to be more powerful, which would in turn lead to increased weight, wires would have to be thicker, other electrical elements would have to be designed to take the increased current/load, etc. All this, just so passengers could enjoy IFE and a hot meal during their short diversion to a nearby airfield.

Technically the VFSG are 250kVA (kilo VoltAmps). VoltAmps and Watts are only equal assuming a perfect power factor.  The various systems are largely motors and controllers and do not have a particularly good power factor, so there is some losses there.
As you noted, the VFSGs are direct coupled to the engines, so they vary in rotational speed based on engine speed, so there are some engine speeds where peak power may not be available.
I don't have wattage numbers for the various systems, but on the 787 everything is electrical, so there are a lot of loads you might not think of.
Some of the larger loads:

Wing Anti-ice
Backup hydraulic pumps for both left and right systems, and both primary and backup pumps for the center system
Pressurization and ECS (climate control)
Nitrogen Inerting system
Fuel pumps
Cargo heaters
Galley systems.

The generators are also used for starting the engines, so if a engine fails in flight the generators on the other side may be called upon to provide power to restart the opposite side while at the same time providing power to the dead engine's hydraulic system, the center system and all the loads above.


text:
How does a HUD track a runway?
I was looking at the FCOM of the 787, and I came across an interesting thing. There is an HUD takeoff selection on the SID selection page. It got me to wonder, how does the HUD track the runway from so far away?
Is it related to ILS?


text:
Why can't the flex temperature be lower than Tref? Also why can't we perform a flex takeoff on a contaminated runway?
This is a huge grey area and my quest to find answers for them is never ending. I have referred to many performance books but to no avail. I hope this question can be clarified. Thanks

I suggest you put the question in the body of your posting as well as the title. In other words, make the body complete. Also, "flex temperature" appears to be unique to Airbus and Fokker. A possibly more common term, I think, at least historically is "assumed temperature takeoff" or "reduced power takeoff."  A short answer to your 2nd question is: Contaminants cause additional drag on the runway (how much you have no way of knowing). Reduced power assumes a clean runway. Use reduced power on a contaminated runway and you might not have enough power to takeoff.

I would suggest asking these questions separately.

The engine is flat rated below Tref, and the thrust decreases above this temperature. The point of using a flex takeoff is to run the engine at a lower power setting, where its limit would be at a higher temperature. Using a temperature below Tref would result in the same thrust as Tref. The graph from Airbus below helps to illustrate the relationship between the variables.

Source

I still didn't quite understand the concept.

Using a flex temperature lower than Tref does not make sense because it would not have any effect, I'll explain my point. The diagram shown before by fooot is pretty nice to see it so I'll refer to it.
After Tref we can see that the maximum thrust decreases, it is due to the temperature increasing and all the adverse effects that it usually have on an engine. However, below Tref we do not see the actual maximum thrust that the engine can produce, or else it would keep on changing in relation to the temperature. What we see is the maximum thrust the engine can produce without suffering structural damages.
I'll quote the Skybrary article about Flex temperature: "At low altitudes and cooler outside air temperatures (OAT), engine pressure is the limiting factor as the engine is capable of producing more pressure and, consequently, more thrust than the engine can contain."
Because of this structural limitation, the flight management system (FADEC in this case) will flat the rate at which the engine produces thrust until Tref.
Therefore, the maximum thrust shown on the diagram after Tref is the actual maximum thrust, because the increase in temperature has decrease the perfomances so much that they are below the structural limitations.
In conclusion, selecting a flex temperature below Tref is the same as selecting TOGA, as it is in fact selecting the highest thrust the aircraft can safely produce.
I hope this helps and I hope it is correct!
https://www.skybrary.aero/index.php/Reduced_Thrust_Takeoff


text:
How does flying at ECON computed speeds optimise fuel burn.?
I read in the FCTM GREEN OPERATING PROCUDURES that it’s recommended to fly at ECON computed speeds throughout the flight profile to optimise speed in accordance with the CI.

I am unsure what your actual question is. Are you trying to understand what is ECON speed, or how is it calculated, or how does it relate to the Cost Index, etc.?

The key is the word 'Econ', which has nothing to do with 'Fuel Burn'. The 'Econ' setting is a MIN COST method of operating, taking into account not just the cost of the fuel, but also the other operating costs of the airplane.
If your hourly costs (maintenance, crew) are high, then you'd burn fuel to get there fast with reduced hours. If your hourly costs are low, then it's worth flying slow, and thus conserving fuel.


text:
Why is there a drift value between the IRS position and the FMGC position present even after performing a full IRS alignment on the A320?
I have noticed this many times across my company's fleet of A320's that is even after a full IRS alignment has been completed many a times there is still a drift value present between the FMGC position and the IRS position. Could any help explain as to how this happens ?

TLDR: The FMGC position is a mix of GPS and IRS (called GPIRS), but the IRS position is aligned to the airport reference coordinate, not to the GPS position.

The difference you observe is between the FM position and the MIX IRS position. This difference is called the bias.

Position Computation
Each FMGC computes its own aircraft position (called the "FM position") from a MIX IRS position (see below), and a computed radio position or GPS position.
The FMGS selects the most accurate position, considering the estimated accuracy and integrity of each positioning equipment.
GPS/INERTIAL is the basic navigation mode provided GPS data is valid and successfully tested. Otherwise, navaids plus inertial or inertial only are used.

(Airbus A320 FCOM - Auto Flight - Flight Management)
The FM position will move towards the GPS position in flight, if GPS is available (otherwise to the radio position):

FM Position
At flight initialization, each FMGC displays an FM position that is a mixed IRS/GPS position (GPIRS).

At takeoff, the FM position is updated to the runway threshold position, as stored in the database, possibly corrected by the takeoff shift entered on the PERF TO page.
In flight, the FM position approaches the radio position, or the GPS position, at a rate that depends upon the aircraft altitude.

Note: The FM position update at takeoff is inhibited when GPS PRIMARY is active.

Bias
Each FMGC computes a vector from its MIX IRS position to the radio or GPIRS position. This vector is called the "bias". Each FMGC continuously updates its bias, if a radio position, or a GPIRS position is available.

(Airbus A320 FCOM - Auto Flight - Flight Management)
The MIX IRS position is determined from all three IRSs:

MIX IRS Position
Each FMGC receives a position from each of the three IRSs, and computes a mean-weighted average called the "MIX IRS" position.

(Airbus A320 FCOM - Auto Flight - Flight Management)
I think your confusion comes from the fact that you think the IRS is aligned with the GPS position and therefore MIX IRS and GPIRS should agree directly after alignment. This is however not true:

Initialization
The F-PLN origin airport coordinates are extracted from the FMS database. These
coordinates appear on the MCDU INITA page, and are normally used for initialization.
They are the airport reference coordinates.
If a high navigation performance is desired, (i.e. for long-range flights without GPS
and without radio navigation updates, or if low RNP operation is expected), the crew
should adjust the airport reference coordinates to the gate coordinates, provided that
this data is published or available on board.

(Airbus A320 FCTM - Normal Operations - Pre Start - ADIRS Initialization)
As you can see the IRS is aligned against whatever is in the MCDU INITA page (airport coordinate by default), not GPS. Therefore, the IRS position can deviate from the FMGC position (GPIRS), even right after alignment.
This deviation will remain in flight, only the FM position will move towards GPS and the bias vector will describe the difference.

I re checked this occurs in a minority of my companies fleet. Typically A320s which are over 10 years old. But in majority of the fleet the aircrafts are equipped with automatic position initialisation for the IRS. That is if GPS is available the initialisation is automatic using the GPS position. So in that case shouldn't the drift value be close to 0 or ar 0?

The default position for alignment is the airport reference point, which is slightly different from the position at the gate, so before performing the alignment, refine your position to match the GPS position using the up/down arrows then press the align key and your drift will be zero zero zero for all IRS at the gate.

From my understanding the FMGC position is a mix from GPS, IRS and radio navigation signals and maybe even other values, e.g. re-calibration to the runway database position as you apply takeoff thrust "on the runway". (I think I read this somewhere, not sure if it is true).
And as far as I know the IRS is aligned only with the GPS position, which could explain a discrepancy between the two values even right after an alignment.
Because the IRS on its own has a constant drift over time the FMGC computes a correction vector for each GPIRS. Maybe these drift corrections are not reset during the alignment (assumed installation error) or they are reset and take a bit of time to be re-evaluated after alignment. They could be an integrator type correction that accumulates the error of the GPS and IRS positions over time to "trim away" the IRS position error. The IRS could drift during the alignment which might only corrected for after the alignment is completed. This would take several minutes before the drift is compensated for.


text:
Can there be more than one airway with the same name in the world?
I was working on a programmed simulation of an FMC. I was wondering, regarding airways, if there can be more than one airway with the same name in the world.
i.e can there be more than one airway named UL602?

Yes.
It may not be ICAO, but there are several military aerial refueling routes with same-name civilian routes.
AR11, AR14, and AR24 are the ones I've worked with. These are aerial refueling routes in Nebraska and Wyoming. There are civilian routes with the same names in Canadian airspace.
These names confused our simulation software, so we loaded the routes as point-to-point. ERAM had no problems with discerning them, and it's unlikely that civilian flight planning software would, either.

Working on it...

That could be, but I'm sure it was listed as an airway. I have to wait until I'm back in the building to check it out. That could be a while, since I'm teleworking full time, only go in to get my badge renewed.


text:
what's the difference between priority and sequence valve in aircraft?
What roles do they play during retraction and extension of a landing gear ?

could you provide some source of where you have encountered the terms?

A priority valve will manage the flow in a hydraulic supply that feeds two different circuits such that if the input flow, that is normally enough to satisfy both circuits' demands, drops below a threshold, the valve will starve or cut off the low priority circuit in favour of the high priority circuit to keep it supplied.  So if I have two subcircuits in a system running on the same source, and I determine that one is critical to safety, I'll put in a priority valve to make sure the critical circuit continues to get pressure if the inlet flow is not able to supply both.
Not to be confused with shuttle valves in brake systems that block off one inlet when pressure is applied at the other inlet.
Sequence valves require a mechanical condition (or electrical condition in the case of a solenoid sequence valve) to be satisfied before supplying or cutting pressure to a circuit.  Like to control gear doors that need to be timed to open or close when the gear legs are at position X or Y.


text:
What is this abandoned plane?
Does anyone knows what type of plane is this and its model?

(Source: YouTube channel AssombradO.com.br)

What is the source of this picture?

Ron Beyer I took it from a YouTube channel intro, but they never metioned nothing about it...

Looks too long forward of the wings for a DC-3.

Which YouTube channel? Unless you personally took the picture, you've acquired a suitable license, or you know for a fact that it is in the public domain, then if you want to reproduce it as you're doing here the very least you can do is link to the place where you got it.

E.P. as the picture was used by the channel without any credits or info, I thought it was public domain and that I could do the same :) And answering your question, it was AssombradO.com.br channel.

And as I said, it was on YouTube in public view, so there's no harm in posting it as far as they do the same every video but in a bigger scale for everyone to see...


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Why is FAC 1 cycled OFF to ON during a dual engine failure?
I was reading in the Airbus A320 FCTM regarding the dual engine failure procedure. The ECAM tells us to recycle FAC 1 OFF THEN ON. Why do we have to recycle FAC 1 specifically? I opened the QRH and it shows this also.


text:
Why is there a difference in the max spoilers extension between the A320 and A321 in a roll?
I was recently reading the Flight Controls chapter in the FCOM and I read that spoilers 2-5 extend a maximum of 35 degrees on the A320 however on the A321 spoilers 2,4 & 5 extend a max of 35 degrees but spoiler 3 extends only a max of 7 degrees (during a roll).
What is the reason behind this?

A321-100 or A321-200?

@JanHudec my company operates the A321-271NX and A321-251NX.


text:
How do partial- and full flaps compare for gusty approaches?
From an aerodynamic aspect, how do partial flaps and full flaps compare for gusty approaches?

What does the manual say?

How do you rate what’s best? Best in what sense?

I fly my archer with 2 notches instead of 3 on gusty days. I yank the last notch in on short final, I find it helps with the yawing a bit. Maybe add 5 knots to my approach speed as well.

My question is mainly in general as the aerodynamic principal will remain the same for all fixed wing aircrafts.

I can't provide a full answer, but I do know that a good part of the benefits of flying with less than full flaps is that you are then required to fly at a faster airspeed, which increases the effectiveness of your control surfaces and you are better able to respond to the gusts.

FWIW, for 747-100/200 aircraft at the two carriers I worked for, all landings were done at 30 degree flaps regardless of the approach conditions.

I’m not sure when it comes to large aircraft, but when you select partial flaps or opt for a no-flap approach and landing, it results in a higher speed on approach and landing.
If you then break down the wind components in vectors, the cross-wind component becomes relatively smaller, so it is as if you have less cross-wind and don’t need to have such a large cross-track angle upon landing (less crab) or less cross-control when touching down. Even though the effect is small, it is noticeable.
You can compare it with a small single-engine piston trying to stay on course with 30° of crosswind at 100 kts needing perhaps 15-25° of cross-track angle while a fighter flying at 450 kts only needs 3-5° or so under the same conditions.

It varies from airframe to airframe, though often times OEMs recommend partial flaps and a gust factor be added to Vref in order to ensure positive control all the way through the approach and roundout - at least for light aircraft.


text:
When did airlines start using seat belts for passengers?
When did airlines start using seat belts on passenger flights?

The world's first airline used Zeppelins, and there was no need for seatbelts.
In WW I seatbelts were standard issue for pilots, at least in Germany.
After the war, the airlines used seatbelts irregularly. Below are two pictures of the cabin of the Junkers F-13, the first all-metal passenger aircraft, which was first operated in 1919.

Junkers F-13 cabin, front view (picture source)

Junkers F-13 cabin, rear view (picture source)
While the belts on the front seats are clearly visible, the rear seats seem not to have them. If the picture date (Oct. 10, 1919) is to be trusted, they were used right from the start in 1919. The way they are fixed to the wicker seats, however, looks rather flimsy and will not give much protection in a crash.

I would guess that the original purpose of seatbelts was (and I think the primary purpose still is) not crash protection, but to keep people in their seats during turbulence.

@jamesqf Agreed!

these appear to have been retrofitted rather than fitted as standard.

@jwenting: Still, look at the date and the use. Also, the soft cover is from the same material as the upholstery. I'm sure not every airline insisted on them - in race cars some drivers preferred not to be buckled in until the Sixties, thinking they would have a higher chance of survival when thrown from the car.

@PeterKämpf those appear to be sleeves fitted around the belts rather than the belts themselves. But yes, it's possible they came with the seats

"The way they are fixed to the wicker seats, however, looks rather flimsy and will not give much protection in a crash." As if the wicker seat itself would hold up well in a crash??? :) If the belt were fixed more firmly, I'd imagine the belt would rip out leaving the bolt with a firm grasp on a now detached chunk of wicker. Otherwise, I'm in complete agreement with your answer!

@FreeMan: Who knows, maybe there is a frame of steel tubing under that wicker? But I guess you are right. Those belts were mainly to keep passengers in their seat in gusty conditions.

I've found photos of Fokker F.IXs from the late 1920s with seatbelts for the passenger seats.  .
The earlier F.VII from 1924 does not appear to have had seat belts.
This more or less frames the introduction of seat belts in airliners in the late 1920s, as Fokker was a leading and innovative airliner manufacturer in those days.
Of course it is always possible that some aircraft were retrofitted even earlier, but this seems to set the start of including them as standard.

Benjamin Foulois (a United States Army general) is credited with inventing the first seat belt on an aircraft in 1910 (Wilbur and Orville: A Biography of the Wright Brothers).

[He] equipped the S.C. No. 1 with the first seat belt, using a four-foot leather cinch obtained from the cavalry saddlery (Wikipedia).

By 1940, seat belts on "transport liners" were common (Flying Magazine, Aug 1940).


text:
Velocity of air flow around aerofoil

To find the velocity of the air flow at each section of the aerofoil, am I right to say that the horizonatal velocity of the air flow is constant throughout the chord while the vertical component of the velocity varies along the chord depending on the angle of chord w.r.t. horizontal?
So velocity at first plane is $\frac{U}{cos\beta}$?
If that is the case, will shifting the air upwards, not push the wing down? Or am I right to say that as the airflow around the wings, there is no net upward motion of air, thus it does not push the wing downwards?

No, you can't say that the horizontal component of velocity is constant throughout the chord. There are few reasons why not:
1) From the formula you provided (which is correct if your assumption on velocity components is true for an aerofoil) we can see that as beta gets bigger the final velocity tends to infinity, which of course cannot happen.
2)The velocity change on aerofoil is dependant upon its pressure change, it reaches maximum at the point of maximum camber and not at the point of maximum thickness and I think that as per your theory it would than be reached at the point with maximum thickness.
3) Third and probably the most important thing is this: Lets imagine the flat plate aerofoil at some small angle of attack. The plate will produce lift, and the particles above an aerofoil will reach the trailing edge first. Since the airflow above aerofoil will be deflected by the same angle as below the aerofoil, by above stated theory, particles should reach the trailing edge at the same time because of a constant horizontal velocity. Which isn't true.
Second part of the question is correct, there is no net upward motion of air. In fact, there is a net downward motion of air behind an aerofoil called downwash. Basically how much the force wing exerts on air by deflecting the partincles down, by that amount of force the aircraft will be pushed up. Newton's third law.

No, horizontal component of velocity varies around the wing, being quite significantly higher above it than below:

(By Kraaiennest [CC BY-SA 3.0  (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons)
Roughly speaking, without involving math, the behaviour of air is like this:

The air avoids the leading edge, and due to inertia would like to continue straight away from the wing.
However viscosity limits the velocity gradient, forcing the air just above and below the wing to move as well¹.
As the air moves out, it frees up space, so pressure decreases.
The oncoming air is accelerated along the pressure gradient, which makes it follow the wing contour.
Decreasing pressure increases velocity due to conservation of energy, as the sum of kinetic and potential¹ energy needs to remain constant; this is known for fluids as the Bernoulli's principle.
Since the air above the wing has to expand to follow the contour, it has lower pressure and correspondingly higher velocity.

Now in mathematical terms, it is the Navier–Stokes equations. They are set of vector partial differential equations. There is no way to solve them analytically, they can only be integrated numerically with help of suitable software. The basic tool for analysing airfoil shapes, reduced to two dimensions, is XFOIL. There is more complex software that can calculate flow around whole aircraft in three dimensions, but unlike free XFOIL, it is either really expensive, or internal tools of large aircraft manufacturers.

¹ Unless the wing stalls. When the flow has low enough energy, the viscosity is no longer enough to flush out the air above the wing and a pocket of stagnant, and thus higher pressure, air forms above the aft part of the wing, which, since it has higher pressure, no longer pulls the wing up generating lift (some lift is still generated on the underside, but it is much less).
² Well, also thermal energy. In adiabatic expansion, as the pressure decreases, the temperature also decreases, which frees up even a little bit more energy to convert to kinetic, but at typical speeds can be neglected.


text:
What is the exact wing chord length and thickness for Boeing 747?
I can't find this info anywhere and all they give me is the ratio. However, I need the exact/approximate values

The chord of a 747 wing (and the thickness) varies along its length. You'll need to be more precise about what you're looking for to get an answer.

I am looking for the approximate length and thus, the mean chord length or the range of chord length will be great.

In general, you can get good general dimensions from the Airplane Characteristics for Airport Planning documents (Boeing). These documents include the overall dimensions and everything relevant for the airport. This does not include the chord length though.
To get more accurate dimensions, you can download CAD drawings from Boeing. I did this for the 747-400 and the 747-8I and measured the chord length in three places along the wing:

As you can see, the chord length varies between 14.84m at the root and 3.70m at the tip (before the winglet). The wing for the 747-8 has been slightly re-designed:

While keeping its basic structure and sweep, the wing is thicker and deeper, holding more fuel, and wider with raked wingtips.

(Wikipedia)
The differences in chord length are rather small (possibly less than the accuracy of these drawings):

I estimate a 565 or 567" chord length where the wing meets the fuselage and wing-to-body (WTB) fairing, based on the Fuselage Station Diagram from the Structural Repair Manual:  https://static.rcgroups.net/forums/attachments/1/8/4/2/1/0/a5165653-56-img063%20-%20Copy.jpg
I would subtract 20" to account for the curvature at the leading edge where the WTB curves and smooths into the leading edge, unless you are going to accurately model that curve.
If you are making a model it'll be helpful to be aware of how the wing is constructed. The primary structure is the aluminum wingbox structure, which takes all the flight loads and spans from wingtip to wingtip, including the center wing section/tank (which is hidden inside the WTB and fuselage).
On a real Boeing this wingbox structure is usually a darker color (except on United Airlines and maybe Singapore and/or Thai and on 747-8s) because it is painted with corroguard.
The leading edge and trailing edge are attached to the wing box along with the flight controls, flaps and everything else.
Over 20 years ago I assembled 747-400 center wing sections and can tell you that a 6' person can stand up inside the first bay of the center wing tank (CWT).
After TWA800 happened the media came through our work area and took pictures:  Here is a photo showing a guy I worked with inside the CWT:
https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse3.mm.bing.net%2Fth%3Fid%3DOIP.AOrngW4VMwMG65-dPj3dgwHaGo%26pid%3DApi&f=1
Here, you'll like this too:  https://static.rcgroups.net/forums/attachments/1/8/4/2/1/0/a5165657-231-img067%20-%20Copy.jpg
Here is NASA's 747 with the wing removed clearly showing the wing box: https://s.hdnux.com/photos/27/13/50/6076974/3/1200x0.jpg
Here are all the photos at rcgroups.net , including more of the 747:
https://www.rcgroups.com/forums/thumbgallery.php?do=threadgallery&t=1734624
Are you making a CAD model or a physical model?  Is it a flying R/C model?
(I just signed up here with my google account to answer this question, so I'm not sure how I will be alerted of your reply, but I assume that I will be).
Matt in Seattle


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Why are/were there almost no tricycle-gear biplanes?
A friend recently asked me: are there any tricycle-gear biplanes? After hunting around, we found the Waco AVN-8, but that's all. This causes me to ask: is there an aerodynamic or other design consideration which makes a tricycle gear a bad choice for a biplane, or is it just an evolutionary coincidence that the move to single-wing designs shortly preceded the move to tricycle gear?

the Super Petrel. Tri-gear, retractable, biplane. (also amphib!)

Here's the ultimate train-wreck of aviation sensibilities-- Belphegor -- https://en.wikipedia.org/wiki/PZL_M-15_Belphegor -- note the tricycle gear

It is a bit more than a coincidence. You might also ask why there are so few biplanes with retractable landing gear. The reason is similar.

Biplanes are propeller-driven (except for this one), and in almost all cases the propeller is in the front. Efficient propellers are large and require a longer landing gear when a tricycle gear is used. Granted, the plane will lift its tail during the ground run so the prop comes down a bit, but that happens over a flat and clean area, so lower tolerances can be afforded during the ground run than during all other ground operations.
Tricycle gears produce substantially more drag. The nose gear sits directly behind the propeller and in its slipstream, so its drag contribution is typically 40% of the whole gear drag. Tricycle gears became less of a drag (pun intended) when gear retraction became standard.
Biplanes are built light and the heavy engine cannot have much distance to the center of gravity. Where do you mount the nose gear? There is preciously little room for another piece of structure. And once that nosegear is crafted on the plane, its distance from the main gear is rather small. Any bump on the field will produce strong pitch oscillations when hit by the nose wheel. Better to have a tailwheel or skid at some distance from the main wheels.
Tricycle gears became a necessity with higher wing loadings and higher landing speeds. Biplanes don't need powerful brakes. On the contrary, on a grass field a taildragger is easier to handle. And with their short ground runs, biplanes often operate from grass fields.

Not that tricycle gears weren't tried. See the picture of a Breguet 6 below (source), conveniently proving that pusher biplanes were also designed. Gun synchronisation removed the last reason not to place the propeller in front.

Re. 3. Most modern single-engine tractor monoplanes have nosewheels, even when retractable. It can't be that hard to find somewhere.

@GuyInchbald This answer is about biplanes.

Precisely. On most twin-engine biplanes you just put the wheels underneath the engines. It is equally easy to do that with single-engined types. Remember, on a bipe they don't generally retract.

The biplane and tailwheel configurations are not directly related, but are both symptoms of the historical quest for light weight.
Given a rather inefficient engine in the 70-150 hp class, lifting both pilot and any kind of payload is a challenge. All-up weight must be severely limited. The biplane is more structurally efficient or, to put it the other way, gives such an aircraft better performance, than a monoplane. Similarly, a tail wheel is simply smaller and lighter than a nose wheel.
200-2,000 hp engines brought the monoplane speeds which the biplane could not reach and so the biplane was all but abandoned. But the need for light weight remained almost as pressing and, with the bulky high-power engine also occupying the space at the nose and pilots already used to tail wheels, the tail wheel remained customary.
But these fast monoplanes had high takeoff and landing speeds and, consequently, high ground speeds before the rudder could take effect. Engine torque did not help matters. As engine powers and takeoff speeds increased rapidly, many a WWII fighter would ground-loop while taxiing or landing and sometimes kill it pilot in the process. The nose wheel was found to be more manageable and by the end of WWII, nose wheels were beginning to appear.
By coincidence, the 1940s also marked the arrival of the jet engine. The prototype Messerschmitt Me 262 had a tailwheel. However the ground handling of those early but unprecedentedly fast Me 262s was nerve-wracking, while suddenly the nose had empty space and a consequent need for noseweight, and engines were becoming so powerful that weight was less of a problem. So Messerschmitt changed over to a nose wheel for the production version. Most of the jets developed during WWII had nose wheels from Day One.
The propeller position, mentioned in some answers, is not especially relevant to biplanes. It is just coincidence that most piston-engines planes, bi- or mono-, are tractors. Pusher biplanes such as the RAF FE2 and Vickers FE8 also had tail wheels as standard (with some also having skids or small nose wheels mounted off the ground on the main u/c structure, in order to prevent nose-over). It was only during WWII that the pusher monoplane was found to be handicapped by the tail wheel and these designs adopted the nose wheel whose star was fortuitously rising. The tail wheel even outlasted the WWII monoplane era and in some cases persisted right through to the postwar jet age. For example Supermarine took another decade to wake up and smell the coffee, as they say, with the swept-wing Supermarine Type 510 and its successors appearing through the late 1940s and even as late as 1950.

The issue with tail wheel on the Me262 was that jet engines have relatively little thrust at slow speeds. Propeller gives a lot of thrust at stand-still which then decreases inversely proportional to speed, while jet gives less thrust at stand-still, but maintains it to high speed so the top speed can be higher, but the take-off run is slower. And the early Me262 had trouble accelerating at all with the drag of the wings at high angle of attack. Switching to nose gear reduced the drag during take-off roll and significantly improved the take-off performance.

Later this required defining the Vr. Comet already had a nose wheel, but when the pilot raised the nose like they were used from propeller aircraft, the performance worsened to the point one crashed. So the procedure was changed to only raise the nose when sufficient speed is attained at which the aircraft will actually lift off.

@JanHudec According to William Green's Warplanes of the Third Reich, the problem with the Me 262 prototypes was ineffective elevators due to the high angle of attack. The tail had to be raised by taking the plane up to 112 mph and then jabbing the wheel brakes to tip the nose forward. That is what I mean by "nerve-wracking" on the ground.

I know that story, but is it really just the jet exhaust affecting the elevators—which is less dense, but also much faster, so I'd expect it not to make them that inefficient—or (also) the high drag from the wings preventing further acceleration? Drag from the high angle of attack was definitely an issue for Comet, so I find it likely it was an issue here too, though there could still be other issues.

As you suspected, this in an evolutionary coincidence. As construction methods and materials improved, multiple stacked wings became obsolete being less efficient to otherwise comparable single wing design.
When biplanes were extinct, by chance the tricycle gear arrangement started its "triumph".  These two are separate advancements in engineering. One could very well combine biplane wing arrangement with tricycle gear, it's just that there's no sense in using a biplane arrangement anymore.

It was more about engine power and airspeed than construction techniques.

Biplanes aren't really extinct, they still have their niche use even today, for example as crop dusters in remote regions. They don't need the efficiency at high speed because they aren't used for traveling, and the extreme low speeds a biplane can provide is an advantage for their role. But as they have to land on rough terrain, (and don't need high speed anyway), a taildragger configuration with non-retractable main wheels is advantageous in their case.

I must admit I may have exaggerated just a tad bit. But they are very rare.

There is another reason, sort of a lost art among modern aircraft designers: to balance vertical and lateral drag.
Early aircraft flew at much slower airspeeds, making them far more susceptible to cross wind gusts.  Large forward set wheels underneath and in front of the fuselage helped balance side wind drag forces on the vertical tail, while cleverly giving the rear vertical airfoil the advantage at lower (yaw) angles of attack.
The biplane configuration also reduces side wind gust roll torque compared with monoplanes of equal wing area.  (Monoplanes solve this with the "high wing" configuration, also seen in paper airplanes).
So, fixed forward raked landing gear really serve an aerodynamic purpose as well (the Fokker DR-1 even put a fourth wing there too).  The ultimate advantage of tricycle gear could be seen with the high speed Space Shuttle landing:  to allow reduction of wing AOA to a non lifting condition while rolling out at high speed.  Higher approach and landing speeds make an aircraft less susceptible to cross wind effects, making this landing technique preferable (in all sorts of weather) provided there is adequate runway length.
One may wonder if the biplane could make a comeback as a recreational ultralight.  One might feel safer with that design as compared with others.

Some of the ideas expressed in this answer might possibly also serve as an answer to this question-- https://aviation.stackexchange.com/q/78013/34686  -- however I find myself a bit skeptical, and also wanting a bit more clarification.  Are you saying there is a specific advantage in terms of in-flight response to crosswind gusts, to putting the (large) wheels as far forward as possible?  Or are you just pointing out that the tailwheel configuration  may tend to be more suited to large mainwheels than the tricycle gear configuration?

@quiet flyer  well, if we remember the sail-like surfaces the Wright brothers put on their aircraft, we can see the difference in drag side torque and airfoil side torque (that the rudder creates).  This means that, in terms of side drag, the aircraft is balanced to a side gust, but the rudder will easily yaw it if applied (airfoil lift is more efficient).